Unfortunately, as usual, there are caveats...
Only Parcel lines have the "spanning" capability, and all parcel lines involved must be in the same Site. That means you often can't use "spanning" to label what you want. Also, in an attempt to help with the "Style Breeding Problem", a couple releases back Autodesk made it so you can also put General Line/Curve labels on Parcel lines, but when you do this, the General label will ALWAYS use an implied "span = false". You cannot get General Line/Curve labels to "span". And you can't put Parcel labels on standard line/curves, so this feature didn't work so well at reducing the "Style Breeding Problem". And Spanning doesn't work over "hard PIs". And "hard PIs" sometimes get created accidentally when editing parcels, but can't be removed without deleting and recreating Parcel segments. But doing THAT will often cause you to lose the assigned Parcel Numbers, Styles, UDPs, etc....
It's a constant string of troubles.